[RTTY] Help!!!

[Steve Matda]

Seems to me that most power supply problems usually occur in four areas: the pass transistors, the regulator (usually an LM723 type), the transistor controlling the pass transistors (usually a TIP41C or similar) and the protection circuits. Sometimes the protection circuits work too well, and hide the real symptoms of the problem.
I'd pull the pass transistors and check them first off with an ohmmeter.  If they are bad, I'd replace them as well as the ballast resistors.    Do you know if the 20 amp rating is peak or continuous?  If it's peak, I'd be immediately suspicious of the pass transistors...  
Working backwards, I would then check the current amp driving the passes...and then last I'd check the operation of the regulator itself to make sure it was driving the current amp correctly. 
That being said, the first thing I do with any supply I get is to socketize the regulator IC if it isn't already...it's then easier to replace when it dies (and trust me, it will). 
As for expense, it really is worth your while to keep a couple of sets of pass transistors, regulators, and current amps handy.  It's hard to believe, but 4 30-amp pass transistors, a regulator, and a current amp runs a total of about 12 dollars.  (Which raises another point--why are power supplies so darn expensive??)
I don't consider myself an expert (and you shouldn't either)...sometimes I'm lucky and fix the problem, and then sometimes I'm left scratching my head amid a cloud of smoke.  This is just the approach I would take.
Hope this helps...from a person who has spent quite a few late nights staring at a schematic and wondering "WHY IN THE #$*% DOESN'T THIS THING WORK??!!!"
Steve, KE4MOB
 Note: forwarded message attached.



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